Add NHS logo for letters

Matches: https://github.com/alphagov/notifications-template-preview/pull/192

migrations/versions/0218_another_letter_org.py

@@ -0,0 +1,35 @@
+"""empty message
+
+Revision ID: 0218_another_letter_org
+Revises: 0217_default_email_branding
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = '0218_another_letter_org'
+down_revision = '0217_default_email_branding'
+
+from alembic import op
+
+
+NEW_ORGANISATIONS = [
+    ('511', 'NHS'),
+]
+
+
+def upgrade():
+    for numeric_id, name in NEW_ORGANISATIONS:
+        op.execute("""
+            INSERT
+                INTO dvla_organisation
+                VALUES ('{}', '{}')
+        """.format(numeric_id, name))
+
+
+def downgrade():
+    for numeric_id, _ in NEW_ORGANISATIONS:
+        op.execute("""
+            DELETE
+                FROM dvla_organisation
+                WHERE id = '{}'
+        """.format(numeric_id))